This invention relates to thermoforming polyolefin resin, and in particular to resins which incorporate an aromatic aldehyde-polyhydric alcohol condensation product and an opacifying pigment.
Thermoforming polyolefin resins entails the steps of heating a sheet of the resin until it softens, stretching the softened sheet against a solid form and allowing the sheet to cool. Heating the sheet is typically accomplished by infrared radiant heaters or by forced convection hot air ovens. The sheet is transported through the heating zone while being suspended by pins mounted on bicycle-style chains running along the sides of the sheet. Alternatively, the sheet may be suspended during transport by tenter-style clamps.
Polyolefin resins are semi-crystalline, that is, upon cooling they form amorphous regions and crystalline regions. Because of the low mobility of polymer chains and the high cooling rates that are typically used to process polyolefins, polymer crystals usually form with varying degrees of crystalline perfection. The crystalline regions act like physical cross-links that hold the polymer together. The crystalline cross-links are the reason that most semi-crystalline polymers can be formed elastically at temperatures above the glass transition temperature of the polymer (T.sub.g). When semicrystalline polymers are heated above T.sub.g, the distribution in crystalline perfection causes the polymer to melt over a range of several degrees. Less perfectly formed crystals have poor thermal stability and are the first to melt. When the first less perfect crystals melt, the physical cross-links begin to break down and the amorphous chains start to relax and flow. At this point the polymer has properties that are ideal for thermoforming; some physical structure exists, but the polymer is easily deformed. At slightly higher temperatures, all of the crystals melt, and the polymer deforms like a viscous liquid.
One of the difficulties encountered in thermoforming resins is maintaining the resin temperature within the narrow processing window, whereby the resin is soft enough to be stretched and shaped, without the loss of the integrity of the sheet. Often under typical processing conditions, the sheet of resin sags or becomes rippled when it is heated. The deformations in the sheet may in turn lead to irregularities in the shaped articles made by the process, such as variations in weight and thickness, anisotropic thermal expansion and shrinkage. The deformation observed in the heated sheet and resulting irregularities may be exacerbated when resins having a relatively high melt flow index are employed. Consequently, resins having a melt flow index of from 1 to 2 are typically used.
The aromatic aldehyde-polyhydric alcohol condensation products employed in the present invention have been incorporated in polyolefin resins as nucleating agents to improve the clarity of the resin. It has been proposed that the condensation products form a network of nucleation sites in the resin upon cooling. Upon crystallization, the resin forms fine spherulites which are smaller than the wavelength of visible light. A description of the nucleating agents may be found in Mannion U.S. Pat. No. 5,310,950, and the references cited therein.
The use of aromatic aldehyde-polyhydric alcohol condensation products as gelling agents for organic solvents is described in Kobayashi et al., U.S. Pat. No. 4,246,000. The condensation products are dissolved in a liquid fuel and castor oil mixture which is used to form a coal-in-oil suspension. The condensation product helps prevent settling of the finely divided coal particles. The aforementioned condensation products have also been employed as gelling agents in cosmetic sticks. For example, Benfatto et al., U.S. Pat. No. 5,376,363 describes a composition containing dibenzylidene sorbitol in an antiperspirant composition. Additionally, the use of dibenzylidene sorbitol to improve the physical property of polyethylene, particularly to increase its tensile strength and raise its melting point, is disclosed in Hamada et al., JP 45-22008 (1970).
A large share of thermoforming is performed with polyolefin resins containing an opacifying amount of pigment. Since the resin is intended to be opaque, there has not been any motivation to incorporate the aromatic aldehyde-polyhydric alcohol condensation products or any other agents which improve resin clarity. Furthermore, the potential benefits for the condensation products with regard to sheet stability during the heating step and facilitation of the use of high melt flow index resins has not been recognized.